Basic Units of Measurement and Conversions - Basic Mathematics for Water and Wastewater Operators - page 84

outgoing land use (i.e., forestland) plus the carbon stocks from one year of growth
in the incoming land use (i.e., cropland), or the carbon in biomass immediately
after the conversion minus the carbon in biomass prior to the conversion plus the
carbon stocks from one year of growth in the incoming land use (i.e., cropland)
(IPCC, 2006). The carbon stock in annual cropland biomass after 1 year is 5 met-
ric tons carbon per hectare, and the carbon content of dry above-ground biomass is
45% (IPCC, 2006). Therefore, the carbon stock in cropland after 1 year of growth
is estimated to be 2.25 metric tons carbon per hectare (or 0.91 metric tons carbon
per acre).
The averaged reference soil carbon stock (for high-activity clay, low-activity
clay, and sandy soils for all climate regions in the United States) is 40.83 metric
tons carbon per hectare (USEPA, 2012b). Carbon stock change in soils is time
dependent, with a default time period for transition between equilibrium soil organ
carbon values of 20 years for mineral soils in cropland systems (IPCC, 2006).
Consequently, it is assumed that the change in equilibrium mineral soil organic
carbon will be annualized over 20 years to represent the annual flux. The IPCC
(2006) guidelines indicate that there are insufficient data to provide a default
approach or parameters to estimate carbon stocks in perennial cropland.

Calculations for Converting U.S. Forests to U.S. Cropland
• Annual change in biomass carbon stocks on land converted to other land-

use category:

ΔC B = ΔC G + C Conversion - ΔC L

where
ΔC B = Annual change in carbon stocks in biomass due to growth on land con-
verted to another land-use category (i.e., 2.25 metric tons C per hectare).
ΔC G = Annual increase in carbon stocks in biomass due to growth on land
converted to another land-use category (i.e., 2.25 metric tons C per hectare).
C Conversion = Initial change in carbon stocks in biomass on land converted to
another land-use category; the sum of the carbon stocks in above-ground,
below-ground, deadwood, and litter biomass (-88.47 metric tons C per
hectare). Immediately after conversion from forestland to cropland, bio-
mass is assumed to be zero, as the land is cleared of all vegetation before
planting crops.
ΔC L = Annual decrease in biomass stocks due to losses from harvesting, fuel
wood gathering, and disturbances on land converted to other land-use cat-
egory (assumed to be zero).

Therefore, ΔC B = ΔC G + C Conversion - ΔC L = -86.22 metric tons carbon per hectare
per year of biomass carbon stocks are lost when forestland is converted to cropland.

• Annual change in organic carbon stocks in mineral soils

ΔC Mineral = (SOC O - SOC ( O-T ) ) ÷ D